NADH Dehydrogenase-like Complex Research Articles

NADH dehydrogenase-like complex L subunit improves salt tolerance by enhancing photosynthetic electron transport

Cyclic electron transport (CET) around photosystem I (PSI) mediated by the NADH dehydrogenase-like (NDH) complex is closely related to plant salt tolerance. However, whether overexpression of a core subunit of the NDH complex affects the photosynthetic electron transport under salt stress is currently unclear. Here, we expressed the NDH complex L subunit (Ndhl) genes ZmNdhl1 and ZmNdhl2 from C4 plant maize (Zea mays) or OsNdhl from C3 plant rice (Oryza sativa) using a constitutive promoter in rice. Transgenic rice lines expressing ZmNdhl1, ZmNdhl2, or OsNdhl displayed enhanced salt tolerance, as indicated by greater plant height, dry weight, and leaf relative water content, as well as lower malondialdehyde content compared to wild-type plants under salt stress. Fluorescence parameters such as post-illumination rise (PIR), the prompt chlorophyll a fluorescence transient (OJIP), modulated 820-nm reflection (MR), and delayed chlorophyll a fluorescence (DF) remained relatively normal in transgenic plants during salt stress. These results indicate that expression of ZmNdhl1, ZmNdhl2, or OsNdhl increases cyclic electron transport activity, slows down damage to linear electron transport, alleviates oxidative damage to the PSI reaction center and plastocyanin, and reduces damage to electron transport on the receptor side of PSI in rice leaves under salt stress. Thus, expression of Ndhl genes from maize or rice improves salt tolerance by enhancing photosynthetic electron transport in rice. Maize and rice Ndhl genes played a similar role in enhancing salinity tolerance and avoiding photosynthetic damage.

Deficiency in NDH-cyclic electron transport retards heat acclimation of photosynthesis in tobacco over day and night shift.

In order to cope with the impact of global warming and frequent extreme weather, thermal acclimation ability is particularly important for plant development and growth, but the mechanism behind is still not fully understood. To investigate the role of NADH dehydrogenase-like complex (NDH) mediated cyclic electron flow (CEF) contributing to heat acclimation, wild type (WT) tobacco (Nicotiana tabacum) and its NDH-B or NDH-C, J, K subunits deficient mutants (ΔB or ΔCJK) were grown at 25/20°C before being shifted to a moderate heat stress environment (35/30°C). The photosynthetic performance of WT and ndh mutants could all eventually acclimate to the increased temperature, but the acclimation process of ndh mutants took longer. Transcriptome profiles revealed that ΔB mutant exhibited distinct photosynthetic-response patterns and stress-response genes compared to WT. Metabolite analysis suggested over-accumulated reducing power and production of more reactive oxygen species in ΔB mutant, which were likely associated with the non-parallel recovery of CO2 assimilation and light reactions shown in ΔB mutant during heat acclimation. Notably, in the warm night periods that could happen in the field, NDH pathway may link to the re-balance of excess reducing power accumulated during daytime. Thus, understanding the diurnal cycle contribution of NDH-mediated CEF for thermal acclimation is expected to facilitate efforts toward enhanced crop fitness and survival under future climates.

Regulatory NADH dehydrogenase-like complex optimizes C4 photosynthetic carbon flow and cellular redox in maize.

C4 plants typically operate a CO2 concentration mechanism from mesophyll (M) cells into bundle sheath (BS) cells. NADH dehydrogenase-like (NDH) complex is enriched in the BS cells of many NADP-malic enzyme (ME) type C4 plants and is more abundant in C4 than in C3 plants, but to what extent it is involved in the CO2 concentration mechanism remains to be experimentally investigated. We created maize and rice mutants deficient in NDH function and then used a combination of transcriptomic, proteomic, and metabolomic approaches for comparative analysis. Considerable decreases in growth, photosynthetic activities, and levels of key photosynthetic proteins were observed in maize but not rice mutants. However, transcript abundance for many cyclic electron transport (CET) and Calvin-Benson cycle components, as well as BS-specific C4 enzymes, was increased in maize mutants. Metabolite analysis of the maize ndh mutants revealed an increased NADPH : NADP ratio, as well as malate, ribulose 1,5-bisphosphate (RuBP), fructose 1,6-bisphosphate (FBP), and photorespiration intermediates. We suggest that by optimizing NADPH and malate levels and adjusting NADP-ME activity, NDH functions to balance metabolic and redox states in the BS cells of maize (in addition to ATP supply), coordinating photosynthetic transcript abundance and protein content, thus directly regulating the carbon flow in the two-celled C4 system of maize.

Two cyclic electron flows around photosystem I differentially participate in C4 photosynthesis.

C4 plants assimilate CO2 more efficiently than C3 plants because of their C4 cycle that concentrates CO2. However, the C4 cycle requires additional ATP molecules, which may be supplied by cyclic electron flow (CEF) around photosystem I. One CEF route, which depends on a chloroplast NADH dehydrogenase-like (NDH) complex, is suggested to be crucial for C4 plants despite the low activity in C3 plants. The other route depends on proton gradient regulation 5 (PGR5) and PGR5-like photosynthetic phenotype 1 (PGRL1) and is considered a major CEF route to generate the proton gradient across the thylakoid membrane in C3 plants. However, its contribution to C4 photosynthesis is still unclear. In this study, we investigated the contribution of the two CEF routes to the NADP-malic enzyme subtype of C4 photosynthesis in Flaveria bidentis. We observed that suppressing the NDH-dependent route drastically delayed growth and decreased the CO2 assimilation rate to approximately 30% of the wild-type rate. On the other hand, suppressing the PGR5/PGRL1-dependent route did not affect plant growth and resulted in a CO2 assimilation rate that was approximately 80% of the wild-type rate. Our data indicate that the NDH-dependent CEF substantially contributes to the NADP-malic enzyme subtype of C4 photosynthesis and that the PGR5/PGRL1-dependent route cannot complement the NDH-dependent route in F. bidentis. These findings support the fact that during C4 evolution, photosynthetic electron flow may have been optimized to provide the energy required for C4 photosynthesis.

Distinct contribution of two cyclic electron transport pathways to P700 oxidation.

Cyclic electron transport (CET) around Photosystem I (PSI) acidifies the thylakoid lumen and downregulates electron transport at the cytochrome b6f complex. This photosynthetic control is essential for oxidizing special pair chlorophylls (P700) of PSI for PSI photoprotection. In addition, CET depending on the PROTON GRADIENT REGULATION 5 (PGR5) protein oxidizes P700 by moving a pool of electrons from the acceptor side of PSI to the plastoquinone pool. This model of the acceptor-side regulation was proposed on the basis of the phenotype of the Arabidopsis (Arabidopsis thaliana) pgr5-1 mutant expressing Chlamydomonas (Chlamydomonas reinhardtii) plastid terminal oxidase (CrPTOX2). In this study, we extended the research including the Arabidopsis chlororespiratory reduction 2-2 (crr2-2) mutant defective in another CET pathway depending on the chloroplast NADH dehydrogenase-like (NDH) complex. Although the introduction of CrPTOX2 did not complement the defect in the acceptor-side regulation by PGR5, the function of the NDH complex was complemented except for its reverse reaction during the induction of photosynthesis. We evaluated the impact of CrPTOX2 under fluctuating light intensity in the wild-type, pgr5-1 and crr2-2 backgrounds. In the high-light period, both PGR5- and NDH-dependent CET were involved in the induction of photosynthetic control, whereas PGR5-dependent CET preferentially contributed to the acceptor-side regulation. On the contrary, the NDH complex probably contributed to the acceptor-side regulation in the low-light period but not in the high-light period. We evaluated the sensitivity of PSI to fluctuating light and clarified that acceptor-side regulation was necessary for PSI photoprotection by oxidizing P700 under high light.

Heat-induced modifications of photosynthetic electron flows in Hordeum vulgare leaves of different age

Temperature is one of the major environmental constraints that limits photosynthetic activity, affecting plant growth and productivity. One of the unresolved issues in studies of thermal adaptation of plants is the question of the difference in adaptive strategies to heat of the photosynthetic apparatus at different stages of development. The aim of this study was to investigate how photosynthetic electron flows respond to hyperthermia (40 °C, 3 h) at different stages of leaf development (four-, seven- and eleven-day-old primary leaves of Hordeum vulgare L.; young, mature and aging, respectively). Chlorophyll fluorescence parameters (including imaging), characteristics of the redox state of P700 and plastoquinones as well as transcription of ndhA and ndhF genes were examined. It was shown that heat inhibited the ferredoxin‐plastoquinone reductase (FQR)-dependent cyclic electron flow (CEF) in all tested variants. In eleven-day-old leaves, an inhibition of the linear electron flow (LEF) was also detected. Heat-induced decrease of CEF and LEF in seven- and eleven-day-old plants (but not in four-day-old seedlings) was compensated by the activation of ‘NADH dehydrogenase-like complex’ (NDH)-dependent electron flow as well as increase of ndhA and ndhF transcription (genes encoding NDH). A hyperthermia decreased the level of plastoquinone pool reduction in all tested leaves. In seven- and eleven-day-old leaves, exposure to heat decreased the photoactive and increased the non-photoactive plastoquinone pools, respectively. Infiltration of leaves by 2,6-dichlorophenolindophenol (DPIP) prevented the decrease of the plastoquinone reduction. It also inhibited the FQR-dependent CEF and qE changes in all age groups. At the same time, the DPIP treatment did not affect the heat-induced suppression of LEF and the redistribution of plastoquinones from the photoactive to the non-photoactive pool in the eleven-day-old leaves. Based on the data obtained, it can be concluded that a decrease in the efficiency of electron donation from plastocyanin to PSI and electron acceptance by ferredoxin could be the reason of heat-induced suppression of FQR-dependent CEF, while a decrease in the size of photoactive plastoquinone pool is potentially a reason of heat-induced LEF inhibition.

The supercomplex formation between the chloroplast NADH dehydrogenase-like complex and photosystem I

The chloroplast genomes of land plants encode 11 genes (ndhA–ndhK) for proteins similar to the subunits of respiratory complex I (NADH dehydrogenase) with some exceptions. It was a mystery why the chloroplast genomes encode the genes related to respiratory electron transport. We understand now that the genes encode the subunits of the chloroplast NADH dehydrogenase-like (NDH) complex mediating the antimycin A-resistant pathway of cyclic electron transport around photosystem I (Peltier et al., 2016).

Supramolecular assembly of chloroplast NADH dehydrogenase-like complex with photosystem I from Arabidopsis thaliana

Cyclic electron transport/flow (CET/CEF) in chloroplasts is a regulatory process essential for the optimization of plant photosynthetic efficiency. A crucial CEF pathway is catalyzed by a membrane-embedded NADH dehydrogenase-like (NDH) complex that contains at least 29 protein subunits and associates with photosystem I (PSI) to form the NDH–PSI supercomplex. Here, we report the 3.9 Å resolution structure of the Arabidopsis thaliana NDH-PSI (AtNDH–PSI) supercomplex. We constructed structural models for 26 AtNDH subunits, among which 11 are unique to chloroplasts and stabilize the core part of the NDH complex. In the supercomplex, one NDH can bind up to two PSI–light-harvesting complex I (PSI–LHCI) complexes at both sides of its membrane arm. Two minor LHCIs, Lhca5 and Lhca6, each present in one PSI–LHCI, interact with NDH and contribute to supercomplex formation and stabilization. Collectively, our study reveals the structural details of the AtNDH–PSI supercomplex assembly and provides a molecular basis for further investigation of the regulatory mechanism of CEF in plants.

OTP970 Is Required for RNA Editing of Chloroplast ndhB Transcripts in Arabidopsis thaliana.

RNA editing is essential for compensating for defects or mutations in haploid organelle genomes and is regulated by numerous trans-factors. Pentatricopeptide repeat (PPR) proteins are the prime factors that are involved in RNA editing; however, many have not yet been identified. Here, we screened the plastid-targeted PLS-DYW subfamily of PPR proteins belonging to Arabidopsis thaliana and identified ORGANELLE TRANSCRIPT PROCESSING 970 (OTP970) as a key player in RNA editing in plastids. A loss-of-function otp970 mutant was impaired in RNA editing of ndhB transcripts at site 149 (ndhB-C149). RNA-immunoprecipitation analysis indicated that OTP970 was associated with the ndhB-C149 site. The complementation of the otp970 mutant with OTP970 lacking the DYW domain (OTP970∆DYW) failed to restore the RNA editing of ndhB-C149. ndhB gene encodes the B subunit of the NADH dehydrogenase-like (NDH) complex; however, neither NDH activity and stability nor NDH-PSI supercomplex formation were affected in otp970 mutant compared to the wild type, indicating that alteration in amino acid sequence is not necessary for NdhB function. Together, these results suggest that OTP970 is involved in the RNA editing of ndhB-C149 and that the DYW domain is essential for its function.

Architecture of the chloroplast PSI-NDH supercomplex in Hordeum vulgare.

The chloroplast NADH dehydrogenase-like (NDH) complex is composed of at least 29 subunits and has an important role in mediating photosystem I (PSI) cyclic electron transport (CET)1-3. The NDH complex associates with PSI to form the PSI-NDH supercomplex and fulfil its function. Here, we report cryo-electron microscopy structures of a PSI-NDH supercomplex from barley (Hordeum vulgare). The structures reveal that PSI-NDH is composed of two copies of thePSI-light-harvestingcomplex I (LHCI) subcomplexand one NDH complex. Two monomeric LHCI proteins, Lhca5 and Lhca6, mediate the binding of two PSI complexes to NDH. Ten plant chloroplast-specific NDH subunits are presented and their exact positions as well as their interactions with other subunits in NDH are elucidated. In all, this study provides a structural basis for further investigations on the functions and regulation of PSI-NDH-dependent CET.

CAF Proteins Help SOT1 Regulate the Stability of Chloroplast ndhA Transcripts.

Protein-mediated RNA stabilization plays profound roles in chloroplast gene expression. Genetic studies have indicated that chloroplast ndhA transcripts, encoding a key subunit of the NADH dehydrogenase-like complex that mediates photosystem I cyclic electron transport and facilitates chlororespiration, are stabilized by PPR53 and its orthologs, but the underlying mechanisms are unclear. Here, we report that CHLOROPLAST RNA SPLICING 2 (CRS2)-ASSOCIATED FACTOR (CAF) proteins activate SUPPRESSOR OF THYLAKOID FORMATION 1 (SOT1), an ortholog of PPR53 in Arabidopsis thaliana, enhancing their affinity for the 5′ ends of ndhA transcripts to stabilize these molecules while inhibiting the RNA endonuclease activity of the SOT1 C-terminal SMR domain. In addition, we established that SOT1 improves the splicing efficiency of ndhA by facilitating the association of CAF2 with the ndhA intron, which may be due to the SOT1-mediated stability of the ndhA transcripts. Our findings shed light on the importance of PPR protein interaction partners in moderating RNA metabolism.

Plastid RNA editing reduction accompanied with genetic variations in Cymbidium, a genus with diverse lifestyle modes

Recent sequencing efforts have broadly uncovered the evolutionary trajectory of plastid genomes (plastomes) of flowering plants in diverse habitats, yet our knowledge of the evolution of plastid posttranscriptional modifications is limited. In this study, we generated 11 complete plastomes and performed ultra-deep transcriptome sequencing to investigate the co-evolution of plastid RNA editing and genetic variation in Cymbidium, a genus with diverse trophic lifestyles. Genome size and gene content is reduced in terrestrial and green mycoheterotrophic orchids relative to their epiphytic relatives. This could be partly due to extensive losses and pseudogenization of ndh genes for the plastid NADH dehydrogenase-like complex, but independent pseudogenization of ndh genes has also occurred in the epiphyte C. mannii, which was reported to use strong crassulacean acid metabolism photosynthesis. RNA editing sites are abundant but variable in number among Cymbidium plastomes. The nearly twofold variation in editing abundance is mainly due to extensive reduction of ancestral editing sites in ndh transcripts of terrestrial, mycoheterotrophic, and C. mannii plastomes. The co-occurrence of editing reduction and pseudogenization in ndh genes suggests functional constraints on editing machinery may be relaxed, leading to nonrandom loss of ancestral edited sites via reduced editing efficiency. This study represents the first systematic examination of RNA editing evolution linked to plastid genome variation in a single genus. We also propose an explanation for how genomic and posttranscriptional variations might be affected by lifestyle-associated ecological adaptation strategies in Cymbidium.

The Physiological Functionality of PGR5/PGRL1-Dependent Cyclic Electron Transport in Sustaining Photosynthesis.

The cyclic electron transport (CET), after the linear electron transport (LET), is another important electron transport pathway during the light reactions of photosynthesis. The proton gradient regulation 5 (PGR5)/PRG5-like photosynthetic phenotype 1 (PGRL1) and the NADH dehydrogenase-like complex pathways are linked to the CET. Recently, the regulation of CET around photosystem I (PSI) has been recognized as crucial for photosynthesis and plant growth. Here, we summarized the main biochemical processes of the PGR5/PGRL1-dependent CET pathway and its physiological significance in protecting the photosystem II and PSI, ATP/NADPH ratio maintenance, and regulating the transitions between LET and CET in order to optimize photosynthesis when encountering unfavorable conditions. A better understanding of the PGR5/PGRL1-mediated CET during photosynthesis might provide novel strategies for improving crop yield in a world facing more extreme weather events with multiple stresses affecting the plants.

Whole-Genome Duplication Facilitated the Evolution of C4 Photosynthesis in Gynandropsis gynandra.

In higher plants, whole-genome duplication (WGD) is thought to facilitate the evolution of C4 photosynthesis from C3 photosynthesis. To understand this issue, we used new and existing leaf-development transcriptomes to construct two coding sequence databases for C4Gynandropsis gynandra and C3Tarenaya hassleriana, which shared a WGD before their divergence. We compared duplicated genes in the two species and found that the WGD contributed to four aspects of the evolution of C4 photosynthesis in G. gynandra. First, G. gynandra has retained the duplicates of ALAAT (alanine aminotransferase) and GOGAT (glutamine oxoglutarate aminotransferase) for nitrogen recycling to establish a photorespiratory CO2 pump in bundle sheath (BS) cells for increasing photosynthesis efficiency, suggesting that G. gynandra experienced a C3–C4 intermediate stage during the C4 evolution. Second, G. gynandra has retained almost all known vein-development-related paralogous genes derived from the WGD event, likely contributing to the high vein complexity of G. gynandra. Third, the WGD facilitated the evolution of C4 enzyme genes and their recruitment into the C4 pathway. Fourth, several genes encoding photosystem I proteins were derived from the WGD and are upregulated in G. gynandra, likely enabling the NADH dehydrogenase-like complex to produce extra ATPs for the C4 CO2 concentration mechanism. Thus, the WGD apparently played an enabler role in the evolution of C4 photosynthesis in G. gynandra. Importantly, an ALAAT duplicate became highly expressed in BS cells in G. gynandra, facilitating nitrogen recycling and transition to the C4 cycle. This study revealed how WDG may facilitate C4 photosynthesis evolution.

Evolution of an assembly factor-based subunit contributed to a novel NDH-PSI supercomplex formation in chloroplasts

Chloroplast NADH dehydrogenase-like (NDH) complex is structurally related to mitochondrial Complex I and forms a supercomplex with two copies of Photosystem I (the NDH-PSI supercomplex) via linker proteins Lhca5 and Lhca6. The latter was acquired relatively recently in a common ancestor of angiosperms. Here we show that NDH-dependent Cyclic Electron Flow 5 (NDF5) is an NDH assembly factor in Arabidopsis. NDF5 initiates the assembly of NDH subunits (PnsB2 and PnsB3) and Lhca6, suggesting that they form a contact site with Lhca6. Our analysis of the NDF5 ortholog in Physcomitrella and angiosperm genomes reveals the subunit PnsB2 to be newly acquired via tandem gene duplication of NDF5 at some point in the evolution of angiosperms. Another Lhca6 contact subunit, PnsB3, has evolved from a protein unrelated to NDH. The structure of the largest photosynthetic electron transport chain complex has become more complicated by acquiring novel subunits and supercomplex formation with PSI.

Identification of the AG afterglow thermoluminescence band in the cyanobacterium Synechocystis PCC 6803.

The so-called afterglow, AG, thermoluminescence (TL) band is a useful indicator of the presence of cyclic electron flow (CEF), which is mediated by the NADH dehydrogenase-like (NDH) complex in higher plants. Although NDH-dependent CEF occurs also in cyanobacteria, the AG band has previously not been found in these organisms. In the present study, we tested various experimental conditions and could identify a TL component with ca. +40°C peak temperature in Synechocystis PCC 6803 cells, which were illuminated by far-red (FR) light at around -10°C. The +40°C band could be observed when WT cells were grown under ambient air level CO2 , but was absent in the M55 mutant, which is deficient in the NDH-1 complex. These experimental observations match the characteristics of the AG band of higher plants. Therefore, we conclude that the newly identified +40°C TL component in Synechocystis PCC 6803 is the cyanobacterial counterpart of the plant AG band and originates from NDH-1-mediated CEF. The cyanobacterial AG band was most efficiently induced when FR illumination was applied at -10°C and its contribution to the total TL intensity declined when cells were illuminated above and below this temperature. Based on this phenomenon we also conclude that CEF is blocked by low temperatures at two different sites in Synechocystis PCC 6803: (1) Below -10°C at the level of NDH-1 and (2) below -30°C at the donor or acceptor side of Photosystem I.

Critical Role of NdhA in the Incorporation of the Peripheral Arm into the Membrane-Embedded Part of the Chloroplast NADH Dehydrogenase-Like Complex.

The chloroplast NADH dehydrogenase-like (NDH) complex mediates ferredoxin-dependent plastoquinone reduction in the thylakoid membrane. In angiosperms, chloroplast NDH is composed of five subcomplexes and further forms a supercomplex with photosystem I (PSI). Subcomplex A (SubA) mediates the electron transport and consists of eight subunits encoded by both plastid and nuclear genomes. The assembly of SubA in the stroma has been extensively studied, but it is unclear how SubA is incorporated into the membrane-embedded part of the NDH complex. Here, we isolated a novel Arabidopsis mutant chlororespiratory reduction 16 (crr16) defective in NDH activity. CRR16 encodes a chloroplast-localized P-class pentatricopeptide repeat protein conserved in angiosperms. Transcript analysis of plastid-encoded ndh genes indicated that CRR16 was responsible for the efficient splicing of the group II intron in the ndhA transcript, which encodes a membrane-embedded subunit localized to the connecting site between SubA and the membrane subcomplex (SubM). To analyze the roles of NdhA in the assembly and stability of the NDH complex, the homoplastomic knockout plant of ndhA (ΔndhA) was generated in tobacco (Nicotiana tabacum). Biochemical analyses of crr16 and ΔndhA plants indicated that NdhA was essential for stabilizing SubA and SubE but not for the accumulation of the other three subcomplexes. Furthermore, the crr16 mutant accumulated the SubA assembly intermediates in the stroma more than that in the wild type. These results suggest that NdhA biosynthesis is essential for the incorporation of SubA into the membrane-embedded part of the NDH complex at the final assembly step of the NDH-PSI supercomplex.

Resilient and Sensitive Key Points of the Photosynthetic Machinery of Coffea spp. to the Single and Superimposed Exposure to Severe Drought and Heat Stresses.

This study unveils the single and combined drought and heat impacts on the photosynthetic performance of Coffea arabica cv. Icatu and C. canephora cv. Conilon Clone 153 (CL153). Well-watered (WW) potted plants were gradually submitted to severe water deficit (SWD) along 20 days under adequate temperature (25/20°C, day/night), and thereafter exposed to a gradual temperature rise up to 42/30°C, followed by a 14-day water and temperature recovery. Single drought affected all gas exchanges (including Amax) and most fluorescence parameters in both genotypes. However, Icatu maintained Fv/Fm and RuBisCO activity, and reinforced electron transport rates, carrier contents, and proton gradient regulation (PGR5) and chloroplast NADH dehydrogenase-like (NDH) complex proteins abundance. This suggested negligible non-stomatal limitations of photosynthesis that were accompanied by a triggering of protective cyclic electron transport (CEF) involving both photosystems (PSs). These findings contrasted with declines in RuBisCO and PSs activities, and cytochromes (b559, f, b563) contents in CL153. Remarkable heat tolerance in potential photosynthetic functioning was detected in WW plants of both genotypes (up to 37/28°C or 39/30°C), likely associated with CEF in Icatu. Yet, at 42/30°C the tolerance limit was exceeded. Reduced Amax and increased Ci values reflected non-stomatal limitations of photosynthesis, agreeing with impairments in energy capture (F0 rise), PSII photochemical efficiency, and RuBisCO and Ru5PK activities. In contrast to PSs activities and electron carrier contents, enzyme activities were highly heat sensitive. Until 37/28°C, stresses interaction was largely absent, and drought played the major role in constraining photosynthesis functioning. Harsher conditions (SWD, 42/30°C) exacerbated impairments to PSs, enzymes, and electron carriers, but uncontrolled energy dissipation was mitigated by photoprotective mechanisms. Most parameters recovered fully between 4 and 14 days after stress relief in both genotypes, although some aftereffects persisted in SWD plants. Icatu was more drought tolerant, with WW and SWD plants usually showing a faster and/or greater recovery than CL153. Heat affected both genotypes mostly at 42/30°C, especially in SWD and Icatu plants. Overall, photochemical components were highly tolerant to heat and to stress interaction in contrast to enzymes that deserve special attention by breeding programs to increase coffee sustainability in climate change scenarios.

Recent advances on the structure and function of NDH-1: The complex I of oxygenic photosynthesis

Photosynthetic NADH dehydrogenase-like complex type-1 (a.k.a, NDH, NDH-1, or NDH-1L) is a multi-subunit, membrane-bound oxidoreductase related to the respiratory complex I. Although originally discovered 30 years ago, a number of recent advances have revealed significant insight into the structure, function, and physiology of NDH-1. Here, we highlight progress in understanding the function of NDH-1 in the photosynthetic light reactions of both cyanobacteria and chloroplasts from biochemical and structural perspectives. We further examine the cyanobacterial-specific forms of NDH-1 that possess vectorial carbonic anhydrase (vCA) activity and function in the CO2-concentrating mechanism (CCM). We compare the proposed mechanism for the cyanobacterial NDH-1 vCA-activity to that of the DAB (DABs accumulates bicarbonate) complex, another putative vCA. Finally, we discuss both new and remaining questions pertaining to the mechanisms of NDH-1 complexes in light of these recent advances.

Regulation of electron transport is essential for photosystem I stability and plant growth.

Photosynthetic electron transport is regulated by cyclic and pseudocyclic electron flow (CEF and PCEF) to maintain the balance between light availability and metabolic demands. CEF transfers electrons from photosystem I to the plastoquinone pool with two mechanisms, dependent either on PGR5/PGRL1 or on the type I NADH dehydrogenase-like (NDH) complex. PCEF uses electrons from photosystem I to reduce oxygen and in many groups of photosynthetic organisms, but remarkably not in angiosperms, it is catalyzed by flavodiiron proteins (FLVs). In this study, Physcomitrella patens plants depleted in PGRL1, NDH and FLVs in different combinations were generated and characterized, showing that all these mechanisms are active in this moss. Surprisingly, in contrast to flowering plants, Physcomitrella patens can cope with the simultaneous inactivation of PGR5- and NDH-dependent CEF but, when FLVs are also depleted, plants show strong growth reduction and photosynthetic activity is drastically reduced. The results demonstrate that mechanisms for modulation of photosynthetic electron transport have large functional overlap but are together indispensable to protect photosystem I from damage and they are an essential component for photosynthesis in any light regime.